The present invention relates to a method and a system for laser welding of overlapped metal sheets.
The welding of overlapped metal sheets by means of a laser beam has been proposed since a long time, particularly in the automotive industry, for the production of motor vehicle bodies or parts thereof, in view of the more conventional electric spot welding technique. After a first increasing spread of the laser welding technique approximately starting from 1990 (see for example European Patent EP-B-0 440 001 in the name of Comau), the application of this technology in the automotive industry has seen a stop due to the increasing use of car bodies made of zinc-plated sheets, i.e. sheets coated with a zinc protective layer, for instance deposited by galvanization. Zinc has a melting temperature of 419xc2x0 C. and evaporization temperature of 906xc2x0 C., whereas this steel constituting sheet typically has a melting temperature around 1,530xc2x0 C. To weld the sheets it is necessary to bring the steel of which they are formed to melt, which means that welding is carried out in the presence of zinc vapours. In the case of electric spot welding, these vapours do not give rise to any substantial draw-backs, since the steel of the sheets starts to melt at the contact area between the sheets (because this area is that having the highest electric resistivity) and since the welding electrodes also apply a pressure on the sheets, which causes evacuation from the sheet contact area of any quantities in excess of melting bath and zinc vapours. In the case of laser welding, instead, the melting bath starts to be formed at the outer surface of the sheet against which the laser beam is directed. As soon as the area of the melting bath, which typically assumes a keyhole-like shape, extends down to the contact area between the sheets, the zinc vapours, which have been created meanwhile at this area, escape outwardly passing through the route of lower resistance, which is represented by the melting bath itself. Therefore, sprays of melted metal are created, which jeopardize the quality and stability of the welded joint, as well as the aesthetical appearance of the product obtained.
Various solutions have been already proposed in the endeavour to overcome the above-mentioned draw-back. A first solution lies in modifying the shape of at least one of the sheets in the welding area, so as to define a gap between the overlapped sheets, which can be exploited by the zinc vapours to escape outwardly (see for example U.S. Pat. Nos. 4,682,002 and 4,916,284). A similar solution lies in providing spacing elements between the sheets, again in order to define a gap for evacuation of the zinc vapours (WO-A-99/08829, JP-A-531855). However all the above-mentioned known solutions have the draw-back to require an additional operation with respect to the conventional work cycle, since it is necessary to form the above-mentioned spacing portions in the sheets or anyhow to apply additional spacing elements, which increases time and cost of production.
The object of the present invention is to overcome the above-mentioned technical problem in a simple and efficient manner.
In order to achieve this object, the invention provides a method for laser welding of two overlapped metal sheets,
wherein at least one of the sheets has a protective layer on its surface in contact with the other sheet, said protective layer being made of a material having a vaporization temperature lower than the melting temperature of the metal forming the sheet,
wherein the sheets to be welded are clamped with each other during the welding operation with the aid of clamping means, and
wherein welding is carried out by directing a laser beam to the area to be welded of the clamped overlapped sheets,
characterized in that welding is carried out by holding the sheets slightly spaced apart from each other at the welding area by means of said clamping means, so that the vapours of protective material, which are created during welding can escape through the gap defined by the two sheets.
In other words, the basic idea of the invention goes against a technical prejudice, which can be found in the known art. In all the known welding systems of the above-mentioned type, indeed, clamping means are provided, which are for clamping the sheets in position during the welding operation. The prejudice mentioned above lies just in that the clamping means were always conceived to press the sheets in contact against each other in order to enable welding to be carried out. In the case of the invention, instead, the clamping means for clamping the sheets in their operative condition do not press the sheets against each other, but rather hold the sheets in a slightly spaced position relative to each other. Studies and tests conducted by the applicant have shown that this slight spacing does not jeopardize the possibility to obtain a welded joint of good quality, since in the welding area the gap between the two sheets is filled by the bath of metal melted by the laser beam. At the same time, the gap between the sheets immediately adjacent to the welded areas enables the zinc vapours, which are created during the welding operation, to escape outwardly with no need to pass through the melted bath and therefore without giving rise to any of the above-mentioned welding defects.
According to the invention, at least one of the sheets is welded in said condition spaced apart from the other sheet with the aid of holding means forming part of said clamping means. The holding means are for instance selected among vacuum cups or magnetic-type holding means.
In the preferred embodiment of the invention, the said clamping means include a support defining a reference and supporting surface for a first sheet, and a clamping member, which is movable with respect to the support between an inoperative position and an operative pre-determined position, in which said movable member defines a reference and supporting surface for the second sheet, the supporting surface on the movable member facing in this operative condition, the supporting surface on the support and being spaced apart therefrom of a length greater than the sum of the thickness dimensions of the sheets to be welded.
Also according to the invention, both the sheets are initially positioned, overlapped with each other, on the reference and support surface of the support, with the clamping movable member in its inoperative position, whereupon the clamping movable member is moved to its operative position so as to bring the support surface of the movable member in front of that of the support, with the second sheet still resting on the first sheet and spaced from the support surface of the movable member. The second sheet is finally grasped and brought against said support surface of the movable member with the aid of said holding means, so as to move the second sheet away from the first sheet. The holding means are able to hold the two sheets in this spaced-apart position while the welding operation is carried out, so as to obtain the advantages which have been mentioned above.
Naturally the invention is also directed to the welding system using the method according to the invention, said system comprising clamping means for clamping the sheets to be welded in position during the welding operation, and means for directing a laser beam to the welding area of the clamped overlapped sheets. As already indicated above, the clamping means are arranged for holding the sheets slightly spaced apart from each other at the welding area during the welding operation. To this end, as indicated above, the clamping means include holding means selected among vacuum cups, magnetic-type means or the like, to hold at least one of the sheets in its condition spaced from the adjacent sheets. Also according to what has already been indicated, in the preferred embodiment, the clamping means comprise a support and a clamping member, which is movable relative thereto. In this embodiment, the above-mentioned holding means are for instance associated with the movable member of the clamping means and are movable, when the clamping movable member is in its operative position, between a forward grasping position and a rearward holding position.
Finally, the invention is also directed to a clamping device taken alone, which can be used to clamp in position two overlapped metal sheets to be welded and having the features which have been already indicated above. The holding means associated with the clamping movable member are movable, when the clamping movable member is in its operative position, between a forward grasping position and a rearward holding position, in which they are able to hold the second sheet spaced apart from the first sheet. As already indicated the holding means may be in form of one or more vacuum cups, or may be magnetic-type holding means or the like. In the former case, actuating means may be provided to drive the movement of the suction cup means between the forward position and the rearward position. Alternatively, the suction cup means are provided so as to include a suction cup portion carried at the end of a bellow-like tubular stem serving for communication of the suction cup to a vacuum source, so that when vacuum is communicated to the suction cup to enable the second sheet to be grasped, the bellow-like stem is contracted, causing the movement of the suction cup towards its rearward position, in which the sheet, after it has been grasped, is kept in a condition spaced apart from the other sheet.
Naturally, the possibility is not excluded to provide clamping devices of a completely different type, such as devices provided with grasping means able to engage both the first sheet and the second sheet. It is also possible to provide a clamping device, in which, instead of a support and a movable member, two members are provided, which are both movable, each provided with respective holding means.
Furthermore, also the grasping means can be made in any way different from that has been described above, while the principle remains which is at the basis of the invention, which is that of holding the two sheets clamped in a position slightly spaced apart from each other during the welding operation.